Close

Hack Chat Transcript, Part 1

A event log for Low-Level Analog Measurement Hack Chat

Chris Gammell measures his Analog Life

LutetiumLutetium 07/17/2019 at 20:020 Comments

Chris Gammell11:54 AM
Hello Hack Chat!

Hi Chris - welcome!

Chris Gammell11:55 AM
thanks!

morgan11:55 AM
oh heeeey

Chris Gammell11:55 AM
Hey Morgan

guido.giunchi12:00 PM
Hi!

Looks like we've got a good turnout, so let's kick it off. Welcome to the Hack Chat everyone, thanks for coming, and thanks to Chris Gammell for hosting. When Chris suggested low-level analog measurement as a topic, I jumped at it - figured we'd all love to hear about that.

Take it away, Chris!

Chris Gammell12:01 PM
OK!

Chris Gammell12:01 PM
so uh

Chris Gammell12:01 PM
Analog signals

Chris Gammell12:01 PM
are everywhere!

Michael Welling12:01 PM
how low level are we talking here?

Chris Gammell12:01 PM
obviously you all knew that

Chris Gammell12:01 PM
so maybe I'll step back

Chris Gammell12:01 PM
and mention a bit about my background

Chris Gammell12:01 PM
and why I thought this might be a good topic

Chris Gammell12:01 PM
I went to school for electronics and struggled with it

Chris Gammell12:02 PM
still do, some days

xinort joined  the room.12:02 PM

Chris Gammell12:02 PM
but then I had the opportunity to work at Keithley Instruments in Cleveland OH

Chris Gammell12:02 PM
and that's where I really got an education in electronics

Chris Gammell12:02 PM
that was working on supporting things like the 2400 source meter, 6517B electrometer, 6485 current source, etc

Chris Gammell12:03 PM
of course, that's been a while now since I was ther

Chris Gammell12:03 PM
but the low level measurement stuff sticks with you

Chris Gammell12:03 PM
namely working with signal processing in the analog domain

Chris Gammell12:03 PM
working with op amps, resistors, capacitors, etc

Kelly Heaton12:03 PM
What signal to noise ratio?

Chris Gammell12:03 PM
and there are a lot of things to consider

Chris Gammell12:03 PM
like power cleanliness and such

Chris Gammell12:04 PM
and very in depth topics for test and measurement

Chris Gammell12:04 PM
but that was the genesis of thinking it'd make a good hack chat topic

Chris Gammell12:04 PM
since then, I mostly work on systems that are sensing something in the real world

Chris Gammell12:04 PM
but usually are piping back data over an i2c or SPI bus

Chris Gammell12:04 PM
so there's less direct interaction with analog signals

Chris Gammell12:05 PM
but most people still need to think about things in terms of analog signals and how they look to the chips they're interacting with

Chris Gammell12:05 PM
so I guess there are some general tips and tricks, but not sure if that's what people are interested in

Chris Gammell12:05 PM
or if they have specific questions they'd like me to answer

Chris Gammell12:06 PM
but I'm happy to do either

Chris Gammell12:06 PM
and can fill in as necessary

Chris Gammell12:06 PM
from a day-to-day basis, I think about troubleshooting and how the interaction with the analog side of things impacted me

Dustin Sackett12:06 PM
If you had a "Top X tips and tricks" to recommend, that pertain to this subject, that might be cool!

Kelly Heaton12:06 PM
why don't you give us an example description of a circuit you have used to buffer a signal?

Chris Gammell12:07 PM
so on the tips and tricks side of things

Chris Gammell12:07 PM
I'm always pulling out a scope first and foremost

Chris Gammell12:07 PM
and checking the rails

Chris Gammell12:07 PM
obviously the voltage levels

Chris Gammell12:07 PM
but also how consisten, or "clean", the power supply looks

Chris Gammell12:07 PM
this also usually allows me to check if I've turned the device in question on

Chris Gammell12:07 PM
which is a pretty common problem as well

Chris Gammell12:07 PM
:-D

Dustin Sackett12:08 PM
also, if you had some solid book or other resource recommendations on the subject, maybe both historically, as well as current day

Chris Gammell12:08 PM
once I know the power source is looking good, I'm usually pulling out a schematic and writing down the signals and levels I expect to see, BEFORE probing the circuit

Sylvain Munaut12:08 PM
How do you juge that ? Maybe it's just me, but even measuring a _battery_ I can see noise spikes .... (coming from the environment presumably). So what's the proper way to measure a rail and make sure you see the rail itself and not ... the room.

Chris Gammell12:08 PM
it's important to create some mental models of what you expect to see before you actually start probing around

Phil12:08 PM
How much does power supply cleanliness affect readings e.g. if I'm using an opamp to buffer a signal, what should I do in respect of the opamp power rails?

Chris Gammell12:09 PM
I don't know about you, but my brain will do backflips to say, "That looks right-ish"

Chris Gammell12:09 PM
> How do you juge that ? Maybe it's just me, but even measuring a _battery_ I can see noise spikes ....

Well if it's on a battery and you see spikes, something is wrong

Chris Gammell12:09 PM
so in that case, I'd isolate the battery by itself

Chris Gammell12:09 PM
I expect to not see any spikes

Chris Gammell12:10 PM
but if the battery by itself does...uh...bad battery!

Chris Gammell12:10 PM
also, run away!

Kelly Heaton12:10 PM
@Dustin Sackett Keith Hemingway's "Electronic Designer's Handbook" is clear and has some useful tips

Chris Gammell12:10 PM

https://media1.giphy.com/media/e1Lv6Gvd8bFFC/source.gif

GIPHY

Run Away Monty Python GIF - Find & Share on GIPHY

Discover & share this Run Away GIF with everyone you know. GIPHY is how you search, share, discover, and create GIFs.

Read this on GIPHY

Sylvain Munaut12:11 PM
No, I expect it's coming from the scope or picking up LED lighting or something. But that's the issue, the environment is not noise free and I end up measuring that rather than the rail itself it seems.

Chris Gammell12:11 PM
> How much does power supply cleanliness affect readings e.g. if I'm using an opamp to buffer a signal, what should I do in respect of the opamp power rails?

Could be quite a bit! If you have a bunch of AC noise that gets rectified, it'll look like the DC signal is higher than it actually is

Dustin Sackett12:11 PM
thank you @Kelly Heaton , cheap and looks exhaustive! I will check that out

Chris Gammell12:11 PM
> No, I expect it's coming from the scope or picking up LED lighting or something. But that's the issue, the environment is not noise free and I end up measuring that rather than the rail itself it seems.

Sure, the environment can really impact things

Chris Gammell12:11 PM
but it's all about isolation and mental models

Chris Gammell12:11 PM
what do you expect to see -> What do you actually see -> Why is that happening -> can you isolate it

Kelly Heaton12:12 PM
@duddy it's much easier to understand than the Bible (Art of Electronics by Horrowitz and Hill, which you need to own anyway even just to lift weights)

Chris Gammell12:12 PM
in terms of isolating different parts of the circuit, I'm usually designing in 0 ohm resistor severywhere

Chris Gammell12:12 PM
so I can swap in a low value resistor to look at current

Chris Gammell12:12 PM
or to pull it entirely and isolate that part of the circuit

Phil12:12 PM
Is it enough to take power from an SMPS, run it through a linear regulator and put a decoupling cap on there? Or is there more to do?

Chris Gammell12:12 PM
I was tutoring someone recently and the 0 ohm resistor thing was a new thing to them

Dustin Sackett12:13 PM
@Chris Gammell could you give a general recommendation on the design process for a high end low-level analog circuits?

Kelly Heaton12:13 PM
@Chris Gammell 0 ohm resistor?

Chris Gammell12:13 PM
and of course, we all start somewhere! That's a huge part of my design process now, knowing how to unhook things from the rest of the ciricuit

Seth12:13 PM
0 ohm resistor = SMD jumper ;)

Chris Gammell12:13 PM
> Is it enough to take power from an SMPS, run it through a linear regulator and put a decoupling cap on there? Or is there more to do?

Depends on your application

Chris Gammell12:13 PM
sometimes that's all you have available

Chris Gammell12:14 PM
> 0 ohm resistor = SMD jumper ;)

And that's right! they're called a few different things

Kelly Heaton12:14 PM
@Seth ha

Chris Gammell12:14 PM
but adding in different "options" in a circuit can really help

Chris Gammell12:14 PM
but jumpers are sometimes designed into the PCB

Chris Gammell12:14 PM
I want to clarify that's not what I normally do

Dustin Sackett12:14 PM
@Kelly Heaton have that Bible you speak of! I never heard about it throughout my education, and saw it recommended on here! :)

Chris Gammell12:14 PM
I'll design a 0 ohm resistor into the signal path

Chris Gammell12:14 PM
and then it's an option when i'm troubleshooting

Chris Gammell12:15 PM
> could you give a general recommendation on the design process for a high end low-level analog circuits?

@Dustin Sackett that's a pretty broad ask :-D

Chris Gammell12:15 PM
like...unlimited budget?

Chris Gammell12:15 PM
that's another thing to note about analog

Chris Gammell12:15 PM
most people don't do it because it's expensive as heck

Chris Gammell12:15 PM
like watch Shahriar or Dave Jones or anyone opening test equipment

Chris Gammell12:15 PM
and you'll see things like...a $5 resistor!

Dustin Sackett12:15 PM
well, maybe just a workflow summary

Kelly Heaton12:15 PM
@Chris Gammell I do it because its dirt cheap!

Chris Gammell12:16 PM
I used to work with a 1 TOhm resistor at Keithely

Chris Gammell12:16 PM
it was bananas

Chris Gammell12:16 PM
and it was really accurate too

Chris Gammell12:16 PM
but usually you'll end up paying for good components

Seth12:16 PM
Geeze, a humid room is less than 1Tohm! Hahaha

Chris Gammell12:16 PM
so I suppose another thing is understanding the differences between accuracy, precision and repeatability.

Dustin Sackett12:17 PM
Napkin sketch>Schematic>Simulation>breadboard (? mixed reviews on this)>PCB layout?

Chris Gammell12:17 PM

https://blog.robotiq.com/hs-fs/hub/13401/file-13222732-jpg/images/repeatability-and-accuracy-resized-600.jpg?width=528&height=467&name=repeatability-and-accuracy-resized-600.jpg

ROBOTIQ

Will Patton12:17 PM
Excellent points Chris, "Look for clean", "Check the rails", "Isolate using 0 ohm". Keep going!

Chris Gammell12:17 PM
so let's look at a simple circuit

anfractuosity12:17 PM
that's a cool diagram!

Chris Gammell12:18 PM

https://www.analog.com/-/media/analog/en/landing-pages/technical-articles/optimizing-precision-photodiode-sensor-circuit-design/figure1.png?w=900&la=en

ANALOG

Chris Gammell12:18 PM
like a simple circuit, right?

Chris Gammell12:18 PM
it's basically measuring the current coming out of (going into) a photodiode

Chris Gammell12:18 PM
but in the three things in that circuit, there's not much you can control

Chris Gammell12:19 PM
so if you want a truly accurate circuit

Discussions